clean up several minor bugs

[samellem]

• when using PyType (even with inferred types), returning None instead of a float causes trouble at compilation time in several places; raise errors instead
• converter.pointwise_minimum_two_arguments was incorrectly referenced as converter.pointwise_minimum_two_args in autodp_core
• in mechanism_zoo.StagewiseMechanism, dp_bank.eps_generalized_gaussian was called without unpacking the params dictionary properly
• return an epsilon of np.inf instead of -1 when \delta is insufficiently large for any worst-case privacy loss guarantee